The present invention relates to an apparatus for leading combustion air to a furnace, which apparatus comprises an air port disposed in the furnace. The air port is connected to an air distribution channel positioned outside the tube wall of the furnace, from which the air flows through the air port into the furnace.
Black liquor obtained from the production of chemical pulp is burnt in a recovery boiler. The air required for the combustion of the organic material in the black liquor is fed to the furnace of the boiler from air distribution channels disposed at various levels around the furnace, through the air ports in the wall of the furnace. Usually nozzles are disposed in the openings in the wall so as to direct air into the furnace. The air is most commonly introduced into the furnace at three levels. At the lowest level is the primary air level, above this, is a secondary air level, and the highest level, above the liquor nozzles, is the tertiary air level. There may also be more than three air levels in the boiler.
Air nozzles have been manufactured of different types of steel and are typically welded or fully cast. Nozzles manufactured of different refractory materials are also used. There are basically two structures that may be used in air nozzles. In one of these structures a nozzle is attached to a gas-tight air port opening formed by bending wall tubes, by for example welding a nozzle made of metal plates into the sides of said tubes. The nozzles may also be attached onto the tube panels by a screw joint. The nozzles are conventionally located inside a box filled with refractory material, the outer shell of which is a steel plate. The boxes are connected to the tube walls, and are usually filled with refractory material. The refractory material protects the nozzle structures and causes heat to move out of the nozzle.
In another embodiment, the nozzle is a distinct structure, and does not form a gas-tight or melt-tight structure with the wall of the boiler. This is a distinct disadvantage of this embodiment.
The combustion air is directed into the nozzles from air ducts encircling (surrounding) the boiler. The air ducts usually include air controlling devices for each of nozzles. The air passes through an air guide, and then an air nozzle, into the furnace.
Conventional air nozzles have a tendency to corrode and crack, and therefore require continuous maintenance in recovery boilers (soda and other types of boilers) burning waste liquors (e.g. black liquor) from the forest industry. This is especially true for the primary air nozzles in a recovery boiler. While the maintenance of the nozzles itself is expensive, a much more significant reason why the corrosion and cracking is dangerous is that nozzle damage may creep into the adjacent water-cooled tubes on the walls of the furnace of the boiler. A water leak in these tubes may cause a melt explosion in the boiler, with potentially disastrous consequences.
One significant reason for the above-mentioned corrosion and cracking of the nozzles and their surroundings is the splashing of the melt generated in the furnace into the air ports, especially into the primary air ports (which are located at the lowest level). The main components of the melt in a sulphate process are sodium carbonate and sodium sulphide. The melt splashes, which have a volume of at least several liters, cause rapid heating of the structure of the air port up to the melting point of the melt, so that the melt salts cause corrosion and erosion in the air ports.
Rapid changes in the nozzle temperature also generate thermal fatigue and stress corrosion in the structures of the air port and even in the surrounding tubes of the furnace. Studies have shown that the temperature of the primary air nozzle, when not properly cooled, varies very rapidly. For example, during the first two hours of a four-hour test period, the temperature varied substantially constantly between about 500 and 850.degree. C. For the last two hours, the temperature dropped so that it was between about 350 and 500.degree. C. most of the time, rising occasionally up to about 600.degree. C. The temperature peaks indicate splashing of melt into the air nozzle.
The air ports and structures in the vicinity thereof are typically cooled by the combustion air being fed to the furnace. They get rapidly damaged if the feed of the combustion air from the port in question is interrupted by closing its respective air damper.
Repairs of air nozzles have to be done regularly during shutdowns. The repairs are difficult and laborious to effect. Dismantling of equipment is necessary and the removal of old nozzles is difficult and time-consuming. Therefore, the shutdowns often last a long time, causing losses in production.
According to the present invention is possible to provide an apparatus for feeding combustion air into the furnace in which the air ports and the structures of the wall of the furnace in the vicinity thereof are protected better than in the prior art against the effects of corrosion and temperature changes. The invention protects the air ports of a boiler burning waste liquor from pulp and paper industries (e.g. black liquor), for example the ports of a recovery boiler, from corrosion and cracking. The invention is especially useful in protecting the primary air ports closest to the melt bed, which are exposed to detrimental effects of the melt. In addition, the invention provides an apparatus that is easy to maintain and repair, so that it is possible to decrease repair and shutdown costs significantly.
According to the present invention the lower part of the air port is provided with a protective insert which is made of substantially heat-and-corrosion-resistant material and mounted and positioned in such a way that it is cooled by the air flowing through the port. That is, the apparatus according to the invention is provided with a separate thermal mass protective insert, so that temperature peaks caused by melt splashes can be effectively evened out, and the structures of the port (especially those at the bottom) can be effectively protected against corrosion and cracking. The protective insert is made of substantially corrosion-resistant material, for example of stainless steel or acid-proof steel. The elongated protective insert according to the invention is attached in such a way that it can be detached relatively easily and may be changed, when needed. The protective insert is also attached in such a way that changing it does not damage the point of attachment or the surroundings thereof. Typically, the insert is disposed at the lower part of the nozzle of the air port, so that it is not in contact with the wall tubes and so that the insert does not have to be detached from the wall tubes of the boiler when the insert is replaced.
The protective insert according to the invention is attached, for example by welding it lightly into the air port, in such a way that the temperature peaks do not cause cracking or corrosion of the cooling tubes of the furnace. Using the massive protective insert cooled by combustion air, cooling capacity may be stored up and then used to cool the structure when melt splashes occur. The protective inserts protect the air ports against the immediate attack of melt and, due to their heat capacity, cool a melt splash so that the temperature rise from a melt splash does not affect the structures surrounding the protective insert.
According to one aspect of the present invention, there is provided an apparatus for feeding combustion air into a furnace having a tube wall. The apparatus comprises: An air port disposed in the tube wall, and connected to an air duct through which air flows into and then through the air port, and into the furnace; and a protective insert of heat and corrosion resistant material mounted in the air port and positioned so that it is cooled by the air flowing through the air port. Preferably the protective insert has sufficient thermal mass so as to effectively even out temperature peaks caused by melt splashes from the furnace impacting the vicinity of the air port. For example, the protective insert is made out of highly corrosion resistant steel (such as stainless steel, or acid proof steel), or a like corrosion resistant high thermal conductivity material, and has a volume of between about 40,000-4,000,000 cubic mm (e.g. between about 1-2 million cubic mm).
The air port may be defined by (or the air port further comprises) a nozzle extending into the air duct the nozzle having an interior and a bottom portion; and the protective insert is preferably mounted to the interior bottom portion of the nozzle, and so that it can be easily replaced. The insert is not in contact with the tube walls.
The apparatus preferably further comprises a refractory filled box attached to the tube wall, the air port disposed within the box adjacent the tube wall.
Also, the insert preferably has an upper surface, most remote from the interior bottom portion; and the upper surface is either flat, or preferably convexly curved. The insert extends within the nozzle, away from the tube wall, a linear distance from the box about 100-700 mm, preferably about 150-500 mm. Also, the insert preferably has a height proportional to about 20-50 mm when the air port has a height of about 150-200 mm, and the insert tapers so that it has a smaller cross-sectional area more remote from the box than adjacent the box; that is the insert has a height which tapers so that it has a smaller cross-sectional area more remote from the tube wall than closer the tube wall. The largest cross-sectional area of the insert is typically about 10-50%, preferably about 15-30%, of the cross-sectional area of the air port within the insert.
According to another aspect of the invention, there is provided a recovery boiler for burning black liquor of a pulp and paper mill to produce heat and to recover chemicals therefrom. The recovery boiler comprises: A furnace having a tube wall, and a black liquor inlet. An air port disposed in the tube wall, and connected to an air duct through which air flows into and then through the air port, and into the furnace. A protective insert of heat and corrosion resistant heat conductive material mounted in the air port and positioned so that it is cooled by the air flowing through the air port; and wherein the protective insert has sufficient thermal mass so as to effectively even out temperature peaks caused by melt splashes as a result of black liquor burning in the furnace and impacting the vicinity of the air port.
According to yet another aspect of the invention there is provided a method of recovering heat and chemicals from black liquor produced by the chemical pulping of cellulose, utilizing a recovery boiler having a furnace with a tube wall, and air ports at least some of which contain nozzles leading from the air ports into an air duct, each nozzle having a bottom interior portion. The method comprises the steps of: (a) Causing air to flow from the air duct, through the nozzles and air ports into the furnace to provide combustion air. (b) Feeding black liquor into the furnace to combust with the combustion air to produce heat energy and produce a melt. (c) Mounting protective inserts of heat conductive corrosion resistant material in at least some of the nozzles on the bottom interior portions thereof, the inserts having sufficient thermal mass so that when cooled by air flowing through the nozzles they even out the temperature peaks caused by melt splashing onto structures in the vicinity of the air ports, and thereby increase the life of the air ports and nozzles; and (d) withdrawing melt from the furnace for chemical recovery. Step (c) is preferably practiced by readily releasably mounting the inserts in the nozzles, distinct from the tube wall; and the method comprises the further step of replacing the inserts when worn without disturbing the tube wall or having to replace the nozzles.
It is the primary object of the present invention to provide enhanced longevity for air ports and/or nozzles in recovery boilers, or the like. This and other objects will be clear from the detailed description and from the appended claims.